On Friday, a group of retired judges will step off a tour bus in a ritzy Michigan suburb after three days of barnstorming through corn fields, cities and coal towns in Ohio and Pennsylvania. They carry with them a message.

In courthouses and public squares, they have marked the nation’s 250th anniversary with a dire warning: The rule of law in America is in grave danger. They will deliver a similar message at a library in Grosse Pointe just outside Detroit — the last stop on an extraordinary tour to defend judicial independence and bolster trust in courts.